The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset.
Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio. The shift is generally characterized as comprising three phases: a preparation phase, a torque phase, and an inertia phase.
In the preparation phase, the on-coming clutch is filled in preparation for torque transmission, and the off-going clutch pressure is progressively reduced in preparation for disengagement. In the torque phase, the on-coming clutch gains torque capacity, and the off-going clutch loses torque capacity at a rate that matches the rate of increase in torque capacity of the on-coming clutch, but without a corresponding change in the input speed. The input speed change occurs during the inertia phase, as the on-coming clutch pressure is controlled to decelerate the input shaft, and the off-going clutch is fully released. The off-going clutch is released during the preparation phase, before the on-coming clutch has achieved sufficient torque capacity; this allows the engine to momentarily accelerate the input shaft prior to the inertia phase of the shift, resulting in a loss of output torque which is perceived by the vehicle occupants as a momentary neutral sensation. The off-going clutch is released after the on-coming clutch has achieved sufficient torque capacity; this results in what is known as a tie-up interval during which the on-coming and off-going clutches are working in opposition, resulting in a sharp drop in output torque that is perceived by the vehicle occupants as a momentary braking sensation.
Since the relative timing of the on-coming engagement and the off-going disengagement is critical to achieving a high quality shift, it has been customary to use a uni-directional torque transmitting mechanism, such as a free-wheel clutch, to release the off-going clutch as the torque capacity of the on-coming clutch builds up during the torque phase of the shift, closely approximating the ideal timing. However, free-wheel clutches significantly increase the cost of a transmission, and various electronic control techniques have been developed for achieving clutch-to-clutch upshifts in which an electronic control module controls both the on-coming clutch apply and the off-going clutch release.
Power downshift delays have been shown to decrease customer satisfaction. Power downshift delays in transmissions are increasing due to high fuel economy requirements for early patterns and increasing numbers of gears in transmissions. Typically, there is a required delay for bringing the off-going clutch element from its holding state to a critical level to control the shift.